1. Field of the Invention
The present invention relates to technology which generates a single look-up table by combining a look-up table for color matching and a look-up table for color separation that are used by an image processing apparatus.
2. Description of the Related Art
When forming an image on a printing medium using an image forming apparatus, if color component data representing the image to be printed and color component data that can be interpreted by the image forming apparatus are different, it is necessary to convert the color component data. Converting the color component data includes, for example, performing color matching processing, converting a color space that represents the image (for example, converting from a RGB color space to a CMYK color space; referred to as “color separation processing”), gamma correction, and converting the number of tones of the color component data. In some cases deterioration in image quality occurs due to various factors when converting the color component data.
Japanese Patent Laid-Open No. 9-294212 discloses technology that reduces deterioration in image quality caused by a quantization error that occurs when reducing the number of tones of image data using an LUT (look-up table). According to the technology disclosed in Japanese Patent Laid-Open No. 9-294212, an LUT is provided such that intervals between lattice points become smaller if the density is lowered for a color space of color component data that represents image data.
Color matching processing and color separation processing can be performed using respectively different LUTs. In contrast, by using a single LUT (hereunder, referred to as “combined LUT”) in which an LUT for color matching and an LUT for color separation are combined, color matching processing and color separation processing may be performed at the same time.
A conventional combined LUT will now be described by referring to FIG. 9 to FIG. 13.
FIG. 9 is a view that conceptually illustrates an LUT for color matching processing. In FIG. 9, each lattice point indicates a sample point (input point) of the LUT. FIG. 10 is a conceptual diagram when FIG. 9 is viewed from a gray axis (axis where R=G=B). At the time of color matching processing for image formation, in general the area of L* in the L*a*b* color system is compressed. Therefore, in FIG. 10, the gradient decreases in a dark portion and in a light portion (a portion in which an input value is small and a portion in which an input value is large).
FIG. 11 is a view that conceptually shows an LUT for color separation processing. FIG. 12 is a conceptual diagram of FIG. 11 when viewed from the gray axis. Although the relation between input values and output values is originally as shown by the solid line (curve) in FIG. 12, for an input value that is other than a sample point, a linearly interpolated value as shown by the dashed line in FIG. 12 is output. Therefore, as shown in FIG. 12, sample points are set at intervals that grow smaller as the area of the gradient of curve increases, so that an interpolation error is smaller.
FIG. 13 is a view that conceptually illustrates an interpolation error caused by an interpolation interval of a 3D-LUT after combination according to the conventional method. FIG. 13 illustrates a case in which points obtained by inputting output values (32, 128, 211) obtained from interpolation sample points of a color matching table into a color separation table are linearly interpolated as sample points. In the figure, it is found that an interpolation error with the original color separation table is large between the input values 32-128 in particular. Because of this error, there is a problem that, in color separation processing, adjustment of a slight ink amount of a gray component is thrown out of balance.
As described above, when performing color matching processing and color separation processing using a conventional combined LUT, there is a problem that there are large interpolation errors and the image quality of the image to be formed deteriorates.
Further, although the technology disclosed in Japanese Patent Laid-Open No. 9-294212 takes into consideration decreasing interpolation errors when using a single LUT, it does not take into consideration generating a combined LUT with small interpolation errors.
Furthermore, as a method for solving the problem whereby interpolation errors are generated as described using FIG. 13, although errors can be made smaller by increasing the number of sample points of a combined LUT and a deterioration in image quality can thereby be reduced, the data amount of the combined LUT increases. An increase in the data amount of the combined LUT becomes a particularly serious problem, for example, in a case in which an image forming apparatus with a limited memory capacity performs color matching processing and color separation processing.
The present invention was made in view of the above circumstances. That is, a feature of the present invention is to provide technology that reduces a deterioration in image quality of an image to be formed while suppressing an increase in a data amount of a combined LUT when generating a single combined LUT by combining a LUT for color matching and a LUT for color separation.